Strong permanent magnets, like rare-earth magnets, are used to an increasing extent in large electrical machines, especially in motors and generators. This is due to the increased efficiency and robustness compared to electrical excitation. But regarding practical applications some difficulties occur. The magnet materials corrode very easily and need a high degree of protection. The materials, in particular of rare-earth magnets, are also rather brittle and cannot safely be fixed by bolting alone.
In WO 00/60617 an encapsulated magnet assembly is disclosed, which comprises a non-metallic housing and a magnet disposed within a housing magnet chamber. A housing end cap is fuse bonded to the housing to encapsulate the magnet therein and form an air and fluid-tight seal with the housing. An insulating spacer is interposed between an exposed surface of the magnet and the end cap before assembly and fuse bonding, and is formed from a thermally insulating material to prevent the transmission of thermal energy to the magnet during the fuse bonding process. The housing also includes one or more projections that extend into the magnet chamber and that cooperate with complementary grooves in the magnet to prevent the magnet from rotating in within the chamber.
In US 2005/0116392 A1 a magnet is described which is encapsulated within a canister formed from two cans into a laminated structure. Each can an end wall and a cylindrical side wall. One can additionally includes an annular lip that slidably fits outside the sidewall of the other can with a small gap therebetween. The magnet is inserted into the two cans together with a flowable and curable adhesive. The adhesive is cured to bond the cans together and to also hermetically seal the structure.
In EP 1 420 501 A2 a rotor which comprises magnets and a method of mounting magnets in such rotor is disclosed. The magnets are embedded in the rotor while maintaining the mounting position in a way quite similar to the mounting position of surface-mounted magnets.
The traditional method of mounting permanent magnets on, for instance, a large generator rotor comprises the following steps. First, the extensive surface of the individual magnets needs to be protected. Secondly, the magnets are fixed to a rotor rim by, for instance, gluing. In a last step, the completed rotor with the magnets glued to it has to be wrapped with a fibreglass bandage.
This method has several uncertainties. The surface protection is expensive and due to new technologies it is not proven over a lifetime of e.g. twenty years. Furthermore, the magnets cannot be magnetised in situ. This means that all work is done with magnetised parts. This requires special tools and stringent work controls to avoid hazardous situations. Once mounted on the rotor and covered by the fibreglass bandage the magnets cannot be removed for a magnetisation in case of an irreversible demagnetisation event.
In order to overcome these difficulties, solutions have been developed whereby magnets are manufactured as complete pole pieces. The permanent magnet pole pieces can be used for electrical machines, for example. In a pole piece one or more magnets are fixed by gluing to a steel base plate and are covered with a protective cover. The protective cover can, for instance, be made of stainless steel. The inside of the protective cover is filled with a filling mass to ensure that the magnets will not move inside the protective cover if the glue joint to the base plate gives way. The filling mass may be, for instance, epoxy resin or silicone rubber. Provided that the cover does not allow diffusion of water vapour and the filling mass completely surrounds the magnet, a high degree corrosion protection of the magnet is not required.
This method more or less eliminates the drawbacks of traditional magnet mounting. Expensive surface protection is not required. Further, the magnets can be magnetised after mounting in the pole piece and the magnets can be removed for magnetisation in case of an irreversible demagnetisation event.
A remaining practical difficulty is the attachment of the protective cover to the base plate. Preferably, the protective cover should be welded or soldered to the base plate in order to create an airtight seal that will further reduce the risk of corrosion. However, it is very difficult to avoid contamination of the welding or soldering point by the material of the filling mass and this can compromise the quality of the joint. Furthermore, the heat input from the welding or soldering process can degrade the filling mass and thereby reduce its corrosion protection capacity. Consequently, bolted joints are sometimes used but this obviously does not provide an airtight joint between the protective cover and the base plate.